1. Field of the Invention
The present invention relates to electrical power supplies used to provide electrical power to amplified measurement transducers, and more particularly to an electrical power supply including a floating current source facilitating the use of line-powered circuits operating from a utility a.c. power source.
2. Description of the Related Art
Piezoelectric materials develop surface electrical charges when subjected to a mechanical force. When an applied force distorts or deforms a piezoelectric crystalline structure, electrical charges within the crystalline structure are displaced, and a net electrical charge is developed across opposed surfaces of the crystalline structure. In many cases, the developed electrical charge is directly proportional to the applied force. Piezoelectric materials include quartz, tourmaline, and man-made piezoelectric ceramic materials (e.g., lead zirconate titanate or PZT).
A piezoelectric transducer includes a piezoelectric material as a sensing element to measure, for example, force, pressure, or acceleration. The electrical signals produced by piezoelectric transducers are often conveyed to remote readout devices by multi-conductor cables. Modem piezoelectric transducers include electronic components which form an electrical interface between the piezoelectric transducers and the cables in order to reduce distortion of the electrical signals (e.g., amplitude reduction, noise pickup, etc.) during transmission along the cables.
FIG. 1 is a diagram of an exemplary prior art measurement system 10 including an acceleration transducer (i.e., accelerometer) 12, a power supply 14, and a readout unit 16. Accelerometer 12 produces an electrical signal proportional to an acceleration experienced by accelerometer 12. Accelerometer 12 may be mechanically coupled to a physical structure or unit under test (UUT) undergoing shock or vibration testing. The electrical signal produced by accelerometer 12 is transmitted to readout unit 16. Readout unit 16 may be, for example, an oscilloscope, a data recorder, or a chart recorder.
Accelerometer 12 includes a seismic mass 18, a piezoelectric sensing element 20, and a signal amplifier 22 enclosed within a housing 24. Piezoelectric sensing element 20 may be a piezoelectric crystalline material (e.g., quartz). Seismic mass 18 is mechanically coupled to piezoelectric sensing element 20 such that when accelerometer 12 experiences an acceleration, seismic mass 18 imposes a mechanical force upon sensing element 20 which distorts a crystalline structure of sensing element 20. For example, when accelerometer 12 experiences acceleration a along a defined axis extending through accelerometer 12, seismic mass 18 may impose a compression force F upon sensing element 20 where F=m.multidot.a. Alternately, seismic mass 18 may impose a tension force F within sensing element 20, or a shear stress within sensing element 20.
When accelerometer 12 experiences an acceleration, and seismic mass 18 imposes a mechanical force upon sensing element 20, sensing element 20 produces an electrical signal (e.g., a charge signal or a voltage signal) between an input terminal of signal amplifier 22 and a reference node 26. When sensing element 20 produces a charge signal, signal amplifier 22 may be a charge amplifier which converts the charge signal to a voltage signal. Signal amplifier 22 produces a voltage signal V.sub.S at an output terminal, where a known relationship exists between voltage signal V.sub.S and the electrical signal produced by sensing element 20.
In the embodiment of FIG. 1, voltage signal V.sub.S is transmitted along a first two-conductor cable 28 to power supply 14, and along a second two-conductor cable 30 to an input of readout unit 16. A signal loop is thus formed between accelerometer 12 and readout unit 16. Signal amplifier 22 preferably has a relatively low output impedance in order that other impedances around the signal loop may be kept relatively low. For example, when the output impedance of signal amplifier 22 is reduced, an input impedance of a differential amplifier 32 within readout unit 16 which receives voltage signal V.sub.S may also be reduced. As a result, the amount of noise introduced into voltage signal V.sub.S during transmission from accelerometer 12 to readout unit 16 is reduced. It is noted that power supply 14 may be incorporated into readout unit 16, thus eliminating the second two-conductor cable 30.
Power supply 14 produces a direct current (d.c.) bias voltage V.sub.B and bias current I.sub.B required by signal amplifier 22. Power supply 14 includes a battery 34 and a constant current diode 36 connected in series between the two conductors of the first two-conductor cable 28. Such power supplies for amplified transducers are well known in the art. Battery 34 produces a d.c. voltage, and constant current diode 36 passes constant d.c. current I.sub.b.As shown in FIG. 1, positive bias voltage V.sub.B is developed between the output terminal of signal amplifier 22 and reference node 26, and bias current I.sub.B flows into the output terminal of signal amplifier 22. Signal voltage V.sub.S produced by signal amplifier 22 is superimposed upon bias voltage V.sub.B such that an electrical voltage of (V.sub.B +V.sub.S) exists between the output terminal of signal amplifier 22 and reference node 26. A d.c. blocking capacitor may be inserted in the signal loop between power supply 14 and readout unit 16, or readout unit 16 may include a d.c. level shifter to remove bias voltage V.sub.B.
FIG. 2 is a diagram of one embodiment of signal amplifier 22 according to the prior art. In the embodiment of FIG. 2, signal amplifier 22 includes an n-channel, depletion-mode metal oxide semiconductor (MOS) transistor 40 having a gate terminal G coupled to a first terminal of sensing element 20, a source terminal S coupled to a second terminal of sensing element 20 and reference node 26, and a drain terminal D coupled to the output terminal of signal amplifier 22. MOS transistor 40 is biased into a linear operating region by bias voltage V.sub.B and bias current I.sub.B provided by power supply 14. Connected in a common source configuration as shown in FIG. 2, MOS transistor 40 has a relatively high input impedance, a relatively low output impedance, and amplifies the voltage produced by sensing element 20. Signal voltage V.sub.S produced by MOS transistor 40 reproduces the electrical signal produced by sensing element 20, and is superimposed upon bias voltage V.sub.B such that an electrical voltage of (V.sub.B +V.sub.S) exists between the output terminal of signal amplifier 22 and reference node 26.
FIG. 3 is a graph of the voltage between the output terminal of signal amplifier 22 and reference node 26 versus time. The linear operating region of MOS transistor 40 exists between a maximum voltage V.sub.MAX and a minimum voltage V.sub.MIN. Bias voltage V.sub.B may be about midway between V.sub.MAX and a minimum voltage V.sub.MIN as shown in FIG. 3, allowing for equally-sized positive and negative voltage swings of signal voltage V.sub.S. An electrical voltage of (V.sub.B +V.sub.S) exists between the output terminal of signal amplifier 22 and reference node 26 as described above and shown in FIG. 3.
One reason batteries are often used to generate bias voltage V.sub.B and bias current I.sub.b is to ensure electrical isolation between the two conductors of two-conductor cable 28 and a ground electrical potential G2 existing at power supply 14. Referring back to FIG. 1, housing 24 of accelerometer 12 may be made of metal (e.g., stainless steel) for strength, durability, and long-term transducer reliability as is typical. In this case, metal housing 24 is also electrically conductive. When accelerometer 12 is mechanically coupled to an electrically conductive structure under test, and the electrically conductive structure is connected to a ground electrical potential G1 at the location of the structure (e.g., for safety reasons), housing 24 is also at ground electrical potential G1. As shown in FIG. 1, accelerometer 12 may include an internal electrical connection 38 which electrically connects reference node 26 to housing 24.
Power supply 14 may be some distance (e.g., hundreds or even thousands of feet) away from accelerometer 12, and ground electrical potential G2 may not be equal to ground electrical potential G1. In this case, a voltage V.sub.G12 exists between ground potential G1 and ground potential G2 as shown in FIG. 1. If a negative terminal of battery 34 is connected to electrical potential G2, unwanted voltage V.sub.G12 is introduced into the signal loop, and an input voltage V.sub.IN at the input of differential amplifier 32 includes voltage V.sub.G12 in addition to signal voltage V.sub.S. Voltage V.sub.G12 represents an unwanted "ground loop" noise voltage, and serves to reduce the signal-to-noise ratio at the input of differential amplifier 32.
On the other hand, battery power supplies are undesirable as they have limited capacities, and their delivered electrical voltages decrease with time. Delivered electrical voltages often decrease very rapidly as batteries near an expended or discharged state, often without warning. For example, as battery 34 within power supply 14 nears an expended or discharged state, bias voltage V.sub.B provided by battery 34 decreases and approaches minimum voltage V.sub.MIN of the linear operating region of MOS transistor 40. As a result, the recording of negative excursions of voltage signal V.sub.S by readout unit 16 may be jeopardized.
Electrical equipment items which derive electrical operating power from an available utility alternating current (a.c.) electrical power source (e.g., a nearby electrical receptacle) are termed "line-powered" equipment items. Most line-powered voltage and current sources which might be deployed at the location of power supply 14 require at least one connection to a stable reference electrical potential--ground electrical potential G2. As a result of this connection, voltage V.sub.G12 may easily be introduced into the signal loop as described above. Known techniques for reducing the amount of ground loop noise introduced into the signal loop at power supply 14 due to V.sub.G12 include inductive coupling through transformer windings and optical coupling through optical couplers. Circuits based upon these techniques tend to be complex and difficult to design and build. They also tend to include relatively large numbers of components, and tend to be relatively bulky and expensive. It would thus be desirable to have a relatively simple, small, and inexpensive floating current power supply suitable for providing electrical power to an amplified piezoelectric transducer.